Electronic ballast with low voltage output

ABSTRACT

The present invention provides a high frequency electronic ballast with low voltage output, comprising a high voltage power supply ( 200 ) generating high voltage power ( 234 ) at a voltage and a frequency; a low voltage microcontroller ( 214 ) which is responsive to the voltage and the frequency of the high voltage power ( 234 ) and generates a low voltage drive signal ( 240 ); and a switch ( 216 ) operably connected to the high voltage power supply ( 200 ) and responsive to the low voltage drive signal ( 240 ). The low voltage output of the switch ( 216 ) can be used to drive auxiliary low voltage loads ( 220 ), such as a back-up incandescent lamp ( 116 ). The low voltage output can be responsive to HID lamp power, so that the back-up incandescent lamp ( 116 ) is energized when the HID lamp is not providing illumination.

The technical field of this disclosure is high frequency ballastsystems, particularly, a high frequency electronic ballast with lowvoltage output.

High Intensity Discharge (HID) lamps, such as mercury vapor, metalhalide, high-pressure sodium, and low-pressure sodium, are used for avariety of lighting tasks. As HID lamps have become more popular,electronic ballasts for HID lamps have been developed.

One challenge with electronic ballasts for HID lamps is to provideefficient, well regulated, low voltage power. Electronic ballasts forHID lamps often provide 120 Volt taps for auxiliary power. The tap canbe used to power back-up lighting which is required when waiting for anHID lamp to re-strike after power loss, to power external controls, orto power other devices. Currently, the 120 Volt tap must be connected tothe powered device through a transformer or other external circuitry.Known electronic taps have limited voltage regulation, and may evenreduce the life of the device fed from the auxiliary power due to highcurrent crest factor and high electromagnetic interference.

It would be desirable to have an electronic ballast with low voltageoutput that would overcome the above disadvantages.

One aspect of the present invention provides an electronic ballast withlow voltage output.

Another aspect of the present invention provides an electronic ballastwith low voltage output having improved voltage regulation.

Another aspect of the present invention provides an electronic ballastwith low voltage output providing power with a reduced current crestfactor and less electromagnetic interference.

The foregoing and other features and advantages of the invention willbecome further apparent from the following detailed description of thepresently preferred embodiments, read in conjunction with theaccompanying drawings. The detailed description and drawings are merelyillustrative of the invention, rather than limiting the scope of theinvention being defined by the appended claims and equivalents thereof.

FIG. 1 shows a block diagram of an electronic ballast with low voltageoutput made in accordance with the present invention.

FIGS. 2A-2C show a schematic diagram of a power supply for an electronicballast with low voltage output made in accordance with the presentinvention.

FIG. 3 shows a schematic diagram of a lamp power circuit for anelectronic ballast with low voltage output made in accordance with thepresent invention.

FIGS. 4A-4F show the circuitry of a ballast control circuit for anelectronic ballast with low voltage output made in accordance with thepresent invention.

FIG. 5 shows a block diagram of a low voltage output circuit for anelectronic ballast with low voltage output made in accordance with thepresent invention.

The present invention provides a high frequency electronic ballast withlow voltage output, comprising a high voltage power supply 200generating high voltage power 234 at a voltage and a frequency; a lowvoltage microcontroller 214 which is responsive to the voltage and thefrequency of the high voltage power 234 and generates a low voltagedrive signal 240; and a switch 216 operably connected to the highvoltage power supply 200 and responsive to the low voltage drive signal240. The low voltage output of the switch 216 can be used to driveauxiliary low voltage loads 220, such as a back-up incandescent lamp116. The low voltage output can be responsive to HID lamp power, so thatthe back-up incandescent lamp 116 is energized when the HID lamp is notproviding illumination.

FIG. 1 shows a block diagram of an electronic ballast made in accordancewith the present invention. Some connections between blocks have beenomitted for clarity of illustration. The electronic ballast 100comprises a power supply 110 fed by mains voltage 120, lamp powercircuit 130 supplying high intensity discharge (HID) lamps 140, andballast control circuit 150. The power supply 110 conditions and adjustspower for the electronic ballast 100, the lamp power circuit 130delivers power to the HID lamps 140, and the ballast control circuit 150controls the operation of the electronic ballast 100.

The power supply 110 comprises an electromagnetic interference (EMI)filter 112 on the input of power supply 110, a 120V power supply 114 forpowering a back-up incandescent lamp 116, a power factor correction(PFC) circuit 117, and an auxiliary low voltage power supply 118 forpowering the ballast control circuit 150. The lamp power circuit 130comprises a capacitor bank 134, a resonant half bridge 136, and anignition circuit 138. The ballast control circuit 150 comprises adimming circuit 152, a power factor correction (PFC) control circuit154, a microcontroller circuit 156, a power regulation circuit 158, acurrent regulation circuit 160, and a driver circuit 162.

FIGS. 2A-2C show a schematic diagram of a power supply for an electronicballast with low voltage output made in accordance with the presentinvention. Referring to FIG. 2A, mains voltage is supplied on terminalconnections X1, X2, and X3. The mains voltage can vary from about 180Vto 305V, and is typically about 200V to 277V. The EMI filter 112connected to mains voltage comprises transformer L3; capacitors C1, C2,C4, C6; and bridge rectifier BD1. Circuit protection can be provided byinrush current limiter RT1 and voltage suppression varistor RV1. Theoutput of EMI filter 112 continues to the low voltage power supply asthe Aux_Line power. An auxiliary line voltage is tapped aftertransformer L3 to supply the 120V power supply as the mains voltagesignal Vmains.

Referring to FIG. 2B, the 120V power supply 114 steps down the 200-277volt Aux_Line power to 120 volts to provide power to the back-upincandescent lamp. HID lamps have a low light output during warm-upphase, which occurs for about the first minute after power is supplied.HID lamps also need to cool down before they can be reignited, typicallyfor about 5 to 15 minutes. The back-up incandescent lamp supplieslighting when the HID lamp is not burning or burning at a low lightlevel. The back-up incandescent lamp can be a halogen lamp or any other120V lamp as desired. The 120V power supply 114 is energized anytime theelectronic ballast is energized. An ELON signal from the ballast controlcircuit determines when the 120V power supply 114 supplies power to theback-up incandescent lamp. The ELON signal turns on the light wheneverthe HID lamp power is less than a predetermined setpoint, such as halfnominal HID lamp power, indicating that the HID lamp is not providingsubstantial light.

The 120V power supply 114 comprises a comparator circuit responsive toan Aux_Line voltage signal and providing an Aux Line zero crossingsignal; a 120V microcontroller responsive to the Aux Line zero crossingsignal and an Aux Line voltage amplitude signal, and providing a 120Vdrive signal; and a 120V driver circuit responsive to the 120V drivesignal and providing 120V power to the back-up incandescent lamp. TheELON control signal from the ballast control circuit switches thecomparator circuit and the 120V microcontroller to turn the 120V powerto the back-up incandescent lamp on and off as required.

The full bridge comprising diodes D1, D2, D3, and D4 rectifies the240-277 volt Aux_Line power. The rectified signal provides an Aux_Linereference signal to comparator U1 after being regulated by voltageregulator U2. The rectified signal also provides a variable Aux_Linevoltage signal to the comparator U1 after being scaled by the voltagedivider comprising resistors R1 and R2. The comparator U1 compares theAux_Line reference signal to the Aux_Line voltage signal and provides anAux Line zero crossing signal to the 120V microcontroller U3. The AuxLine zero crossing signal is used to determine the Aux Line frequency.

The Aux_Line power is scaled by the voltage divider comprising resistorsR3 and R4 and provided to the 120V microcontroller U3 as an Aux Linevoltage amplitude signal after further conditioning with diode D5,capacitors C10, and resistors R3, R4.

The 120V microcontroller U3 uses the Aux Line zero crossing signal andAux Line voltage amplitude signal to determine a 120V drive signal fortriac Q1. The 120V microcontroller U3 employs a preprogrammed look-uptable to look-up the desired timing/phase angle of the triac Q1 based onthe Aux Line voltage amplitude signal and corrected for the Aux Linefrequency as indicated by the Aux Line zero crossing signal. The 120Vdrive signal switches the triac Q1 through transformer T1 to provide awell-regulated 120V power to the back-up incandescent lamp. The 120Vpower supply 114 supplies well regulated 120V power, which will increasethe life of the back-up incandescent lamp, and provides over voltageprotection to the back-up incandescent lamp.

The ELON control signal from the ballast control circuit switchesoptical isolator ISO1 to turn the 120V power to the back-up incandescentlamp on and off as required. To turn the 120V power off, opticalisolator ISO1 grounds the reference voltage to comparator U1 and themaster clear pin on the 120V microcontroller U3.

FIG. 2C shows a schematic diagram of a power factor correction and lowvoltage power supply for an electronic ballast made in accordance withthe present invention. The power factor correction circuit 117 receivesthe output voltage of the EMI filter and boosts the power supplied tothe auxiliary low voltage power supply 118 and the lamp power circuit.

The power factor correction circuit 117 provides a high power factor andlow total harmonic distortion. The power factor correction circuit 117adjusts the rail voltage supplying the lamp power circuit with respectto the mains voltage to reduce the power losses, which would occur byholding a fixed rail voltage independent of the mains voltage. Powerfactor correction circuit 117 comprises transformer T2, switch Q3 anddiode D10. The mains voltage signal Vmains is passed through resistorR10 to provide the mains voltage signal Vmains to the PFC controlcircuit in the ballast control circuit. The PFC control circuitprocesses the mains voltage signal Vmains, PFC current signal Ipfc, andPFC voltage signal Vpfc, and returns a PFC gate signal Gpfc to the powerfactor correction circuit 117. The PFC gate signal Gpfc cycles switch Q3so that both output voltage requirements and input current requirementsare met. In one embodiment, the rail voltage Vrail can be set todiscrete values for particular mains voltages. For example, if the mainsvoltage is below about 210-215 volts, the rail voltage can be set toabout 400 volts. Likewise, for mains voltages of about 210 to 255 voltsand above about 250 volts, the rail voltage can be set to about 450volts and about 465-480 volts, respectively. Hysterisis can be used toprevent inadvertent switching of the rail voltage near the mains powervoltage setpoints. Those skilled in the art will appreciate thatdifferent mains voltage ranges and rail voltages can be used as suitedfor particular applications. Transformer T2 also provides a zero currentinput signal ZCin to the PFC control circuit to indicate when current inthe transformer T2 has reached zero. Transformer T2 also provides powerto the dimming circuit in the ballast control circuit through Vdimm+ andVdimm−. The power factor correction circuit 117 provides voltage signalsto the ballast control circuit through the PFC voltage signal Vpfc andthe scaled PFC output voltage signal Vpf.

The auxiliary low voltage power supply 118 provides power to the ballastcontrol circuit components. The auxiliary low voltage power supply 118takes power from the output of the power factor correction circuit 117and produces lower voltage power at 15 volts using switched mode powersupply IC U5. Voltage regulator Q5 regulates the output from theswitched mode power supply IC U5. The output of voltage regulator Q5provides power to the PFC controller through the Vccpfc line and powerto the other ballast control circuit components through the +15 line.

FIG. 3 shows a schematic diagram of a lamp power circuit for anelectronic ballast with low voltage output made in accordance with thepresent invention. The lamp power circuit 130 comprises capacitor bank134, resonant half bridge 136, and ignitor 138. Capacitor bank 134 actsas an energy buffer. The resonant half bridge 136 receives power fromthe EMI filter and converts the power to drive the HID lamp. The ignitor138 provides a high voltage to the HID lamp during lamp startup.

Capacitor bank 134 on the output of the power factor correction circuitcomprises electrolytic capacitors C15 and C16. Resonant half bridge 136comprises switches Q7, Q9, inductor L4, and capacitor C17. The power tothe HID lamp is controlled by the impedance of inductor L4 and capacitorC17, and the frequency of the alternate switching of switches Q7 and Q9in response to high gate signal Hgate and low gate signal Lgate,respectively. High gate signal Hgate and low gate signal Lgate and theirrespective grounds, HSource and LSource, are supplied by the ballastcontrol circuit.

Signals from the resonant half bridge 136 also provide information tothe ballast control circuit. A lamp power signal Psense+ is provided bymeasuring the voltage across resistor R12 to indicate the power input tothe resonant half bridge 136. A sensed lamp current signal Isense+ toIsense− is provided by measuring the current through the transformer T3which is mounted in series with inductor L4 and capacitor C17. Thevoltage for the HID lamp can be determined by dividing the lamp power bythe lamp current.

Ignitor 138 comprises DC offset circuit 139 operably connected to theconnection of inductor L4 and capacitor C17, clamping circuit 137operably connected to a secondary winding on inductor L4, and capacitorC19. The ignition voltage for the HID lamp is generated by the resonancebetween the inductor L4 and the capacitor C19, in conjunction with a DCoffset voltage applied to capacitor C17 by the DC offset circuit 139.The resonance is a first harmonic resonance.

The DC offset circuit 139 comprises diodes D12, D14, D16, capacitorsC21, C23, C25, resistor R14, and diode D18. The DC offset circuit 139provides a DC offset voltage to capacitor C17 to reduce currents inswitches Q7 and Q9 during generation of the ignition voltage. Themagnitude of the DC offset voltage is a fixed ratio of the inductorvoltage. The DC offset voltage is controlled by the resonant voltage oninductor L4, which is determined by the current through inductor L4. Afeedback loop is provided as transformer T3 measures the current throughinductor L4 and provides the sensed lamp current signal Isense+ toIsense− to the ballast microcontroller in the ballast control circuit.The ballast control circuit controls the frequency sweep with the Hgate,Lgate, Hsource, and Lsource signals to switches Q7 and Q9. The DC offsetvoltage can be set between about 1 kV and 2.5 kV depending on theparticular application.

A hardware control/limiting circuit is also provided to control thevoltage on inductor L4. The hardware control/limiting circuit comprisescoil L6, diode D21, capacitor C27, resistor R16, and zener diode D20.The current through coil L6 generates a voltage that is rectified bydiode D21 and filtered by capacitor C27 to produce a voltage controlledoscillator (VCO) feedback signal VCOfb. The VCO feedback signal isprovided to the voltage controlled oscillator (VCO) in the ballastcontrol circuit as a feedback control and limit, allowing the ballastcontrol circuit to control the voltage on inductor L4. In oneembodiment, the coil L6 is a saturating coil to reduce the effects ofswitching of the diodes D24, D25, D26, D27.

The clamping circuit 137 comprises a secondary winding of inductor L4, arectifier bridge of diodes D24, D25, D26, D27, capacitor C29, and diodeD21. The clamping circuit 137 conducts if the secondary winding voltagebecomes too high, thus limiting the voltage at the inductor L4 to therail voltage above circuit ground. The winding ratio of the secondarywinding of inductor L4 can be used to set the voltage at which theclamping circuit 137 conducts.

In another embodiment, an ignition switch (not shown) responsive to anignition signal from the ballast control circuit can be provided inseries with capacitor C19. The ignition switch can allow the ballastcontrol circuit positive control over the ignition of HID lamp based onthe control information provided to the ballast control circuit.

FIGS. 4A-4F show the circuitry of a ballast control circuit for anelectronic ballast with low voltage output made in accordance with thepresent invention. FIG. 4A shows a schematic diagram of a dimmingcircuit for an electronic ballast made in accordance with the presentinvention. An analog dimming signal is a manually or automaticallyadjustable input signal received by the dimming circuit 152 at jack J2.The analog dimming signal can be 0-10 volts, or other voltage ranges asrequired for a particular application. The dimming circuit 152 in theballast control circuit 150 is protected from high input voltage at jackJ2 by positive temperature coefficient (PTC) overcurrent protector RT2and zener diode D30. The analog dimming signal feeds voltage controlledoscillator U9, which converts the analog dimming signal into a frequencydimming signal Dimm with frequency proportional to the analog dimmingsignal voltage. The frequency dimming signal Dimm is fed to opto-couplerISO1, which isolates the dimming circuit 152 output from themicrocontroller circuit. The power factor correction circuit providespower to the dimming circuit 152 through Vdimm+ and Vdimm−, with voltageregulator U7 providing voltage stabilization.

FIG. 4B shows a schematic diagram of a power factor correction (PFC)control circuit 154 in the ballast control circuit 150 for an electronicballast made in accordance with the present invention. Using powerfactor correction U10, the PFC control circuit 154 processes the mainsvoltage signal Vmains, PFC current signal Ipfc, and PFC voltage signalVpfc from the power factor correction circuit, and returns a PFC gatesignal Gpfc to the power factor correction circuit. The PFC controlcircuit 154 receives a zero current input signal ZCin to indicate whenthe current in transformer in the PFC circuit has reached zero.

The target rail voltage for a particular mains voltage range is set bythe resistor array of resistors R20, R21, R22, and R23. The ballastmicrocontroller responds to the mains voltage signal Vmains and suppliessupply power factor voltage signals Vpf_(—)3, Vpf_(—)2, Vpf_(—)1, andVpf_(—)0, which switch the various resistors in the resistor array tocircuit ground. The resistor array supplies different voltagescorresponding to possible rail voltages, which bias the PFC voltagesignal Vpfc to the power factor correction U10.

FIGS. 4C & 4D show a schematic diagram of a microcontroller circuit andballast microcontroller detail, respectively, for an electronic ballastmade in accordance with the present invention. The ballastmicrocontroller U12 is the main control component of the electronicballast and the ballast control circuit. The microcontroller circuit 156receives information on the various parameters throughout the electronicballast and supplies control signals to the various components.Oscillator Y1 provides an oscillating signal, typically about 4 MHz, tothe ballast microcontroller U12. Ballast microcontroller U12 receives 5Vpower from the power regulation circuit, which receives 15V power fromthe auxiliary low voltage power supply. EEPROM U14 stores informationsupplied to the ballast microcontroller U12 to tune the electronicballast to the proper power level, run-up current, and ignition voltage.

The dimming signal Dimm from the dimming circuit is an input to themicrocontroller circuit 156 directing the ballast microcontroller U12 toset the power to the HID lamp by adjusting the power reference signalPref to the power regulation circuit.

The Sweep signal is an output from the microcontroller circuit 156 tothe driver circuit to sweep the frequency and generate the requiredvoltage during ignition. The Sweep signal is a function of the ignitionvoltage signal Vign. The Sweep signal also modulates the lamp currentfrequency during steady state operation to increase arc stability. Thesteady state operation is described in U.S. patent application Ser. No.10/043,586, assigned to the same assignee as the present application andincorporated herein by reference.

The power reference signal Pref is an output from the ballastmicrocontroller U12 and provides the power regulation circuit with thepower reference signal with which to compare the processed sensed powersignal to adjust the output of the HID lamp. The power reference signalPref controls the HID lamp power and is a function of measured railvoltage Vpf and sensed power signal Pwr. The power reference signal Prefcan also be a function of the frequency dimming signal Dimm and thecalibration constant from EEPROM U14. The SCL and SDA signalscommunicate stored information from EEPROM U14, such as power level,run-up current, and ignition voltage, to the ballast microcontrollerU12.

The supply power factor voltage signals Vpf_(—)3, Vpf_(—)2, Vpf_(—)1,and Vpf_(—)0 are outputs from the ballast microcontroller U12 providingcircuit grounds to the resistor array in the PFC control circuit to setthe target rail voltage. The grounding of Vpf_(—)3, Vpf_(—)2, Vpf_(—)1,and Vpf_(—)0 is a function of mains voltage Vmains.

The Tx and Rx signal provide communication between the ballastmicrocontroller U12 and devices external to the electronic ballastthrough port J1 using an RS232 interface protocol.

The input voltage signal Vmains is an input to the ballastmicrocontroller U12 from the PFC control circuit 154 and indicates themains voltage level. The input voltage signal Vmains determines theballast microcontroller U12 setting the output for the supply powerfactor voltage signals Vpf_(—)3, Vpf_(—)2, Vpf_(—)1, and Vpf_(—)0.

The scaled PFC output voltage signal Vpf is an input to the ballastmicrocontroller U12 from the power factor correction circuit 117 andindicates the rail voltage.

The processed power signal Pwr is an input to the ballastmicrocontroller U12 from the power regulation circuit and indicates thepower to the HID lamp. The processed power signal Pwr divided by thelamp current signal Isense+ provides the HID lamp voltage. The processedpower signal Pwr, scaled PFC output voltage signal Vpf, a calibrationconstant from EEPROM U14, and Dimming signal Dimm are used to determinepower reference signal Pref, which controls the HID lamp power.

The temperature signal Ts is an input to the ballast microcontroller U12from the overcurrent protector RT3 of the microcontroller circuit 156and indicates the temperature of the electronic ballast. The temperaturesignal Ts can be used by the ballast microcontroller U12 to determinethat the electronic ballast should be shut down to avoid damage: theballast microcontroller shuts down the electronic ballast by togglingthe shutdown signal SD.

The ignition voltage signal Vign is an input to the ballastmicrocontroller U12 from the ignitor and indicates the voltage suppliedto the HID lamp for ignition. The ignition voltage signal Vign can beused by the ballast microcontroller U12 to determine the magnitude ofthe sweep signal Sweep to start the HID lamp.

The lamp current signal Isense+ is an input to the ballastmicrocontroller U12 from the current regulation circuit, which receivesthe signal from the resonant half bridge. The lamp current signalIsense+ indicates the current to the HID lamp and is used to controlrunup current limit signal Iworm. The lamp current signal Isense+ isalso used to calculate lamp voltage, which can be used for suchfunctions as determining fault situations.

The ballast microcontroller U12 can determine voltage for the HID lampby dividing the processed power signal Pwr by the lamp current signalIsense+. The ballast microcontroller U12 can use the processed powersignal aPwr, current signal Isense+, and calculated HID lamp voltage todetermine the magnitude of power reference signal Pref to control theHID lamp. The power reference signal Pref can also be a function of thefrequency dimming signal Dimm and the calibration constant from EEPROMU14.

The ELON signal is an output from the ballast microcontroller U12 to the120V power supply and determines when the 120V power supply suppliespower to the back-up incandescent lamp. The ELON signal turns off theback-up incandescent lamp whenever the HID lamp power, as indicated bythe Pwr signal to the ballast microcontroller U12, reaches apredetermined setpoint. A predetermined setpoint, such as about 50%nominal HID lamp power, can be used to indicate the point where the HIDlamp provides substantial light.

The run up current limit signal Iworm is an output from the ballastmicrocontroller U12 to the voltage controlled oscillator of the drivercircuit. The run up current limit signal Iworm sets the lamp currentlimit level and is required at low HID lamp voltages to limit run upcurrent. The run up current limit signal Iworm is a function of lampcurrent signal Isense+, which indicates the current to the HID lamp.

The inverse power on signal-Pwr_On is the power up/reset signal forinitializing the ballast microcontroller U12.

The shutdown signal SD is an output from the ballast microcontroller U12to the high and low side driver of the driver circuit. The shutdownsignal SD turns off the HID lamp on fault conditions, such as no lampignition, lamp voltage outside range, ballast temperature high, andmains voltage low.

FIG. 4E shows a schematic diagram of power regulation circuit 158 andcurrent regulation circuit 160 for an electronic ballast made inaccordance with the present invention. The power regulation circuit 158compares a sensed lamp power signal to a power reference signal todetermine a power error signal, which is passed to the currentregulation circuit 160. The current regulation circuit 160 uses thepower error signal and sensed lamp current to determine a total errorsignal, which is passed to the driver circuit 162.

The power regulation circuit 158 includes operational amplifiers U16 andU17. Operational amplifier U16 receives lamp power signal Psense+ whichindicates the power through switch Q9 of the resonant half bridge (seeFIG. 3). Operational amplifier U16 regulates and limits the lamp powersignal to produce a processed power signal Pwr, which is supplied to theoperational amplifier U17 and also to the microcontroller circuit.Operational amplifier U17 compares the processed power signal Pwr to thepower reference signal Pref from the microcontroller circuit to producea power error signal Perr, which is supplied to the current regulationcircuit 160. The power regulation circuit 158 also includes voltageregulator U21 to supply power to the microcontroller circuit.

The current regulation circuit 160 includes operational amplifiers U18and U19. Operational amplifier U18 compares the power error signal Perrto the sensed lamp current signal Isense+ from the resonant half bridgeto produce a power/current error signal PIerr, which is supplied to theoperational amplifier U19. Operational amplifier U19 regulates andlimits the power/current error signal PIerr and produces a total errorsignal Err, which is supplied to the driver circuit.

The sweep signal Sweep from the microcontroller circuit to theoperational amplifier U19 sweeps the frequency and generates therequired voltage during ignition and modulates the lamp currentfrequency during steady state operation to increase arc stability. Thesteady state operation is described in U.S. patent application Ser. No.10/043,586, assigned to the same assignee as the present application,and incorporated herein by reference.

FIG. 4F shows a schematic diagram of a driver circuit 162 for anelectronic ballast made in accordance with the present invention. Thedriver circuit 162 receives the total error signal Err from the currentregulation circuit indicating the desired power to be supplied to theHID lamp and provides high gate signal Hgate and low gate signal Lgateto the resonant half bridge to control power to the HID lamp.

The driver circuit 162 comprises voltage controlled oscillator (VCO)U24, driver gates U26, U27, U28, U29, U30, and high and low side driverU32. VCO U24 receives the total error signal Err from the currentregulation circuit and provides a clocked VCO output signal VCOUTproportional to the voltage of the total error signal Err. The runupcurrent limit signal Iworm or the run shutdown signal SD from themicrocontroller circuit can shut down the VCO U24 to turn off the HIDlamp, if required.

The driver gates receive the VCO output signal VCOUT, which passesthrough three driver gates U26, U27, U28 to produce the high inputsignal Hin and through two driver gates U29 and U30 to produce the lowinput signal Lin. The use of an odd number of driver gates to producethe high input signal Hin and an even number of driver gates to producethe low input signal Lin results in the high input signal Hin and thelow input signal Lin having opposite polarity with deadtime between thetwo signals.

High and low side driver U32 regulates the high input signal Hin and thelow input signal Lin from the driver gates and provides the high gatesignal Hgate and low gate signal Lgate to the resonant half bridge. Therun shutdown signal SD from the microcontroller circuit can shut downthe VCO U24 to turn off the HID lamp, if required.

FIG. 5 shows a block diagram of a low voltage output circuit for anelectronic ballast with low voltage output made in accordance with thepresent invention. The high voltage power from the high voltage powersupply 200 is reduced to a lower voltage to power auxiliary low voltageloads 220, such as a back-up incandescent lamp. If used to power aback-up incandescent lamp, the low voltage output circuit can beresponsive to the HID lamp power to turn the back-up incandescent lampon and off as required. In other embodiments, the low voltage output 218can be used to power other components or circuits, such as an insulationdetector.

In the present example, the high voltage power supply 200 provides about200-277 volts, which is equivalent to the mains voltage. Those skilledin the art will appreciate that the high voltage power supply 200 can beany power supply in the electronic ballast that it is desirable to stepdown to a lower voltage for a particular application, including thepower supply for mains voltage. The voltage of the low voltage output218 can be determined through programming the low voltagemicrocontroller 214 to provide the particular voltage required for aparticular low voltage load 220, such as about 120 volts for a back-upincandescent lamp.

Back up lighting for HID lamp systems is often desirable because HIDlamps have a low light output during the warm-up phase, which occurs forabout a minute after applying power to the HID lamp. HID lamps also needto cool down after they are turned off, typically making themunavailable for about 5 to 15 minutes. A back-up incandescent lampsupplies lighting when the HID lamp is not burning or burning at a lowlight level. The back-up incandescent lamp can be a halogen lamp or anyother 120V lamp as desired.

High voltage power supply 200 provides low voltage output 218 to the lowvoltage load 220 through the switch 216. The switch 216 can be a triac,such as triac Q1 of FIG. 2B, or another switch, such as a high powerswitching transistor or a thyristor. Exemplary triacs are the BTA216 andBT136 manufactured by Philips Semiconductors. Referring to FIG. 5, thelow voltage output 218 to the low voltage load 220 is determined by theswitching of switch 216 in response to the low voltage drive signal 240from low voltage microcontroller 214. The low voltage microcontroller214 uses the voltage and frequency of the high voltage power supply 200to determine the low voltage drive signal 240. High Voltage (HV) voltageamplitude signal 230 from the high voltage power supply 200 indicatesthe voltage of the high voltage power supply 200 to the low voltagemicrocontroller 214. HV zero crossing signal 238 indicates the frequencyof the high voltage power supply 200. Those skilled in the art willappreciate that the voltage and frequency of the high voltage powersupply 200 can be determined by a number of other methods, such asresistive dividers, capacitive circuits, and capacitive dividers.

To produce the HV zero crossing signal 228, the high voltage powersupply 200 supplies high voltage power 234 to rectifier 206, whichconverts the high voltage power 234 to a HV voltage signal 236.Comparator 212 compares a reference voltage signal 208 to the HV voltagesignal 236 to produce an HV zero crossing signal 238 representative ofthe frequency of the high voltage power supply 200. The low voltagemicrocontroller 214 employs a preprogrammed look-up table to look-up thedesired timing/phase angle of the switch 216 based on the HV voltageamplitude signal 230 and corrected for the frequency of the high voltagepower supply 200 as indicated by the HV zero crossing signal 238. Thevalues in the look-up table can be determined by calculation andexperiment, and can account for non-linearity in the system.

The low voltage microcontroller 214 can be responsive to a switchingsignal 232 to switch the low voltage output 218 on and off. A ballastsignal 204 can be provided to the ballast microcontroller 206 to producethe switching signal 232. The ballast signal 204 can be indicative of anumber of parameters inside or outside the ballast depending on theparticular application of the low voltage output circuit. The ballastmicrocontroller 206 can measure the ballast signal 204 againstpredetermined setpoints or electronic ballast parameters to determinethe desired state of the switching signal 232. The low voltagemicrocontroller 214 can be responsive to the switching signal 232 toturn the low voltage drive signal 240 on and off as required. Thoseskilled in the art will appreciate that a number of switching circuitsusing microcontrollers, transistors, mechanical switches, andcombinations thereof, can be used to provide the switching signal 232.

In one present embodiment, the low voltage output circuit is used topower a back-up incandescent lamp as the low voltage load 220, with theballast signal 204 being an HID lamp power signal and the switchingsignal 232 being the ELON control signal. See FIG. 2B. The low voltageoutput circuit can be responsive to the HID lamp power signal to turnthe back-up incandescent lamp on and off as required. The HID lamp powersignal indicates the HID lamp power. The HID lamp power signal isprovided to the ballast microcontroller 206, which compares the HID lamppower signal to a predetermined HID lamp power setpoint, such as about50% nominal HID lamp power, to determine if the back-up incandescentlamp should be on or off. If the HID lamp power is above thepredetermined HID lamp power setpoint, the ballast microcontroller 206sets the ELON control signal so that the low voltage microcontroller 214turns off the low voltage drive signal 240 to the switch 216, turningoff the back-up incandescent lamp. If the HID lamp power is below thepredetermined HID lamp power setpoint, the ballast microcontroller 206sets the ELON control signal so that the low voltage microcontroller 214turns on the low voltage drive signal 240 to the switch 216, turning onthe back-up incandescent lamp.

It is important to note that FIGS. 1-5 illustrate specific applicationsand embodiments of the present invention, and are not intended the limitthe scope of the present disclosure or claims to that which is presentedtherein. Upon reading the specification and reviewing the drawingshereof, it will become immediately obvious to those skilled in the artthat myriad other embodiments of the present invention are possible, andthat such embodiments are contemplated and fall within the scope of thepresently claimed invention.

While the embodiments of the invention disclosed herein are presentlyconsidered to be preferred, various changes and modifications can bemade without departing from the spirit and scope of the invention. Thescope of the invention is indicated in the appended claims, and allchanges that come within the meaning and range of equivalents areintended to be embraced therein.

1. A low voltage output circuit for an electronic ballast comprising: a high voltage power supply (200), the high voltage power supply (200) generating high voltage power (234) having a voltage and a frequency; a low voltage microcontroller (214), the low voltage microcontroller (214) being responsive to the voltage and the frequency of the high voltage power (234) and generating a low voltage drive signal (240); and a switch (216), the switch (216) being operably connected to the high voltage power supply (200) and being responsive to the low voltage drive signal (240).
 2. The low voltage output circuit of claim 1 wherein the high voltage power supply (200) is a power supply for mains voltage (120).
 3. The low voltage output circuit of claim 1 wherein the low voltage microcontroller (214) is responsive to a switching signal (232).
 4. The low voltage output circuit of claim 3 further comprising a ballast microcontroller (206), the ballast microcontroller (206) being responsive to a ballast signal (204) and generating the switching signal (232).
 5. The low voltage output circuit of claim 4 wherein the ballast signal (204) is an HID lamp power signal and the ballast microcontroller (206) switches the switching signal (232) when the HID lamp power signal reaches a predetermined setpoint.
 6. The low voltage output circuit of claim 5 wherein the predetermined setpoint is about 50% nominal HID lamp power.
 7. The low voltage output circuit of claim 1 wherein the switch (216) provides low voltage output (218) to an incandescent lamp (116).
 8. The low voltage output circuit of claim 1 wherein the switch (216) is a triac.
 9. A method of producing a low voltage output for an electronic ballast comprising: providing a high voltage power supply (200), the high voltage power supply (200) generating high voltage power (234); determining voltage of the high voltage power (234); determining frequency of the high voltage power (234); determining a low voltage drive signal (240) based on the voltage and the frequency; and switching a switch (216) operably coupled to the high voltage power supply (200) in response to the low voltage drive signal (240).
 10. The method of claim 9 wherein determining frequency of the high voltage power (234) comprises: rectifying the high voltage power (234) to generate a high voltage (HV) voltage signal (236); comparing the HV voltage signal (236) to a reference voltage signal (208) to generate an HV zero crossing signal (238) indicative of the frequency.
 11. The method of claim 9 wherein determining a low voltage drive signal (240) based on the voltage and the frequency further comprises: providing a low voltage microcontroller (214) having a look-up table; and looking up the low voltage drive signal (240) for the voltage and the frequency on the look-up table.
 12. The method of claim 9 further comprising switching the low voltage drive signal (240) in response to a switching signal (232).
 13. The method of claim 12 further comprising providing a ballast microcontroller (206), the ballast microcontroller (206) generating the switching signal (232) in response to a ballast signal (204).
 14. The method of claim 9 further comprising providing a low voltage output (218) from the switch (216) to an incandescent lamp (116).
 15. The method of claim 14 further comprising: determining power to an HID lamp; and switching off the incandescent lamp (116) when the power to the HID lamp exceeds a predetermined power.
 16. The method of claim 15 wherein the predetermined power is about 50% nominal HID lamp power.
 17. A system for producing a low voltage output for an electronic ballast comprising: means for providing high voltage power (234); means for determining voltage of the high voltage power (234); means for determining frequency of the high voltage power (234); means for determining a low voltage drive signal (240) based on the voltage and the frequency; and means for switching operably coupled to the high voltage power providing means, the switching means being responsive to the low voltage drive signal (240).
 18. The system of claim 17 wherein the frequency determining means further comprises: means for rectifying the high voltage power (234) to generate a HV voltage signal (236); means for comparing the HV voltage signal (236) to a reference voltage signal (208) to generate an HV zero crossing signal (238) indicative of the frequency.
 19. The system of claim 17 wherein the low voltage drive signal (240) determining means further comprises: means for storing a look-up table; and means for looking up the low voltage drive signal (240) for the voltage and the frequency from the look-up table.
 20. The system of claim 17 further comprising means for switching the low voltage drive signal (240), the low voltage drive signal (240) switching means being responsive to a switching signal (232).
 21. The system of claim 17 further comprising means for backup lighting operably connected to the switching means.
 22. The system of claim 21 further comprising: means for determining power to an HID lamp; and means for switching off the backup lighting means when the power to the HID lamp exceeds a predetermined power.
 23. The low voltage output circuit of claim 1 wherein the switch (216) provides low voltage output (218) to an insulation detector.
 24. The method of claim 9 further comprising providing a low voltage output (218) from the switch (216) to an insulation detector.
 25. The system of claim 17 further comprising means for detecting insulation operably connected to the switching means. 